rc4: mark key as const

Key data is never written so the parameter can be const, which allows
putting fixed keys in .rodata.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>